In electrophotography, the development process is carried out by adhering a toner to an electrostatic latent image on a photosensitive member. A general electrophotographic developer contains a toner and a carrier for imparting charge to the toner by frictional charging and carrying the toner to a development region. In general, the toner comprises a thermoplastic resin, a colorant, and a charge controller, and the carrier comprises a magnetic powder of iron, ferrite (iron oxide) or the like. It is known that besides the toner and carrier, various additives can be added to the developer for the purpose of controlling the charge of the toner, improving the fluidity and cleaning properties of the toner, etc.
For example, a hydrophobic metal oxide powder obtained by making hydrophobic the surfaces of a metal oxide powder of silica, alumina, titania or the like using an organic material is added as an agent for improving the fluidity and cleaning properties of the toner to an electrophotographic developer. Such a hydrophobic metal oxide powder is also known to be used as a charge controller. Therefore, hydrophobicity and frictional charge are important features for this type of hydrophobic metal oxide powder, and these features are controlled by the type and the coating amount of the organic materials used for surface treatment of a metal oxide powder.
In general surface treatment of a metal oxide powder, surface treatment of the powder with an amine organic compound is known to decrease the negative charge of the powder or impart positive charge to the powder. It is also proposed that the metal oxide powder surface-treated with such an amine organic compound is used as a charge controller for toner.
For example, Japanese Examined Patent Publication No. 53-22447 proposes that a metal oxide powder surface-treated with aminosilane (an amino-containing silane coupling agent) is used as a positive charge controller for toner. In addition, Japanese Unexamined Patent Publication No. 62-52561 discloses an electrophotographic toner containing as a positive charge controller a powder obtained by treating vapor phase silica with epoxy-containing silane and then with an amine. In general, a metal oxide powder surface-treated with such silane cannot be sufficiently increased in hydrophobicity, and does not have the sufficient effect of improving the fluidity and cleaning properties of the toner.
Japanese Unexamined Patent Publication Nos. 6-80406 and 6-83099 disclose that organopolysiloxane containing epoxy groups and a metal oxide powder surface-treated with polyalkyleneimine are added to an electrophotographic developer for the purpose of both improving fluidity and controlling charge. However, it was found that since the polyalkyleneimine (for example, polyethyleneimine) used as an amine is a polymeric amine containing many amino groups per molecular weight and having a high molecular weight, and the surface of the powder is covered with polyalkyleneimine chains with relatively high hydrophilicity, hydrophobicity and fluidity become insufficient.
Japanese Unexamined Patent Publication No. 63-155155 discloses an electrostatic image developer containing inorganic fine particles which are treated with silicone oil containing epoxy groups and then with an amino compound. However, the inorganic particles treated in two steps as described above have unreliable charge controllability and/or insufficient hydrophobicity.
An object of the present invention is to provide a hydrophobic metal oxide powder which can be controlled in frictional charge within a wide range by using a relatively small amount of coating, and which is provided with high hydrophobicity and fluidity, and also an electrophotographic developer containing the hydrophobic metal oxide powder.